1. Field of the Invention
The present invention relates to a dc-dc converter.
2. Description of the Prior Art
With accelerated global warming, decreasing natural resources, increasing fuel price, and economical issues, vehicles with electric propulsion, such as hybrid electric vehicles (HEVs), plug-in HEVs (PHEVs), battery electric vehicles (BEVs or EVs), and fuel cell electric vehicles, are gradually growing.
These vehicles need commonly rechargeable batteries as the energy source of electric traction system. Among them, PHEVs or EVs require a higher capacity and larger sized battery pack compared with other vehicles because the battery is a main energy source in PHEVs or EVs.
The high-energy-density battery pack in PHEVs or EVs is typically recharged from the ac utility grid via an ac-dc converter named as battery charger. For low harmonic contents on the ac utility grid and high efficiency, most of battery chargers have generally the basic form of an ac-dc converter with a power factor corrector (PFC), followed by an isolated dc-dc converter.
There are key requirements in the development of EV battery chargers. First, it is imperative to reduce their size and weight in order to facilitate packaging and to highlight the utilization factor of energy. Namely, the design for higher power density and lower weight is required. Furthermore, the conversion efficiency should be maximized during whole output conditions or battery recharging process to maximize the fuel saving and emission reduction.
In order to achieve these requirements, it is necessary to adopt higher switching frequencies and soft-switching technologies since a higher switching frequency is the key to reducing the size and weight of passive components used in high-power applications, and soft-switching technologies significantly lower the generated switching losses.
In addition, in the PFC stage, a bridgeless design should be carried out because excessive conduction loss is generated due to the forward voltage drop for each of the bridge diodes, particularly at a lower line input voltage, which decreases the overall efficiency and greatly increases the size and weight of heat sink.
In addition, in the case where the output voltage requirement of the battery charger is high, the rectifier diodes in the dc-dc converter could experience a serious voltage oscillation and spike. Then, lossy snubber circuitry and higher voltage-rated diodes must be required, which cause the increase in power loss, size, and weight. Thus, in designing the rectifier stage in the dc-dc converter, the design that can avoid the aforementioned problem should be also taken into account.
In addition, in order to maintain high efficiency under low power conditions, it is necessary to minimize the amount of circulating energy in the dc-dc converter.
Conventional phase-shift full-bridge (PSFB) converter is the most preferred dc-dc topology for battery charger applications because of natural zero-voltage-switching (ZVS) operation, low current ripple in battery charging current, and simple structure and control.
However, for wide-output-voltage-range applications like battery charger, the conventional PSFB converter does not obtain an optimal efficiency due to narrow ZVS range, large circulating current, and high voltage stress on the rectifier, etc.